Uninterruptible power supplies are utilized in applications requiring a reliable, continuous power supply such that the load does not see any input power outages or disturbances at the input to the power supply. Uninterruptible power supplies achieve this objective by utilizing a primary source of power which is augmented upon demand by a secondary source of power. Three phase uninterruptible power supplies typically achieve this objective by first rectifying the primary source of AC power in each phase to fully charge a battery which is part of a reserve power source. Circuitry is provided in series with the primary power path to invert the battery voltage to generate an output power signal. Due to the large filtering effect of the battery, the load is unaware of primary outages. This is inefficient because the power is processed in two successive steps, namely rectification and inversion.
One type of single phase uninterruptible power supply has overcome the aforementioned disadvantage by utilizing a double-shunted ferroresonant transformer to process the primary source of power in parallel with the secondary source of power.
A typical prior art uninterruptible power supply utilizing parallel power processing which is suitable for single-phase applications is shown in FIG. 1. Shown therein is an uninterruptible power supply utilizing a ferroresonant transformer 10. The primary power source is coupled to a primary power input winding 11. Normally the primary power supply comprises commercial AC supplied by a public utility. Secondary power is applied to the secondary power input winding 12. Normally the secondary power comprises a battery DC voltage source 13 and an inverter 17. The inverter is energized by the DC voltage source 13 and switching of the inverter switching devices is controlled by an inverter drive circuit 14.
A double-shunted ferroresonant transformer is used so that the two different input sources can be coupled in parallel to the output. Both input windings 11 and 12 are coupled to the output winding 16 through the inductances of their respective shunts 15a and 15b.
The flow of real power from either input winding to the output winding is proportional to the sine of the phase angle between the signal at the input and the signal at the output. Hence, by varying this phase angle, the flow of real power from either input winding 11 or 12 to the output winding 16 may be controlled. In the idling condition the inverter is operated so that its output signal is in phase at winding 12 with the power output signal on winding 16 and, hence, no power is delivered by the inverter to the output.
The switching devices in the inverter 17 are driven by the inverter drive circuit 14 which in turn is controlled with respect to the timing of the drive signals by the phase shift control circuit 18 and the system clock 23. The system clock is synchronized with the input AC when the signal is normally available. The phase shift control circuit 18, in the illustrative example of FIG. 1, is connected to a power sense circuit 19 which monitors power flow from the battery or reserve voltage source 13. The phase angle of the inverter output is modified until power flow from the battery 13 is nulled, hence, the inverter operates in an idling condition.
Should the primary power supplied to the primary input winding fail, an input monitor circuit 21 is connected to detect this loss of power and applies a signal to the clock 23 permitting it to run freely, and opens line switch 22, whereby the inverter, acting as a secondary power source, now supplies all power to the output. An example of a single-phase, uninterruptible power supply utilizing parallel power processing in inductively coupled circuits is found in U.S. Pat. No. 4,010,381 issued to H. Fickenscher et al, on Mar. 1, 1977 and assigned to the same assignee as this application.
The uninterruptible power supply system disclosed in the aforementioned Fickenscher patent is very advantageous from an efficiency standpoint. It is apparent to those skilled in the art that a parallel power processing system would be advantageous for use in three-phase uninterruptible power supplies. However, substituting the aforedescribed single phase inductively coupled system into a three-phase supply according to the prior art introduces difficulties in controlling the inverters appropriately in an idling condition. It is apparent to those skilled in the three-phase art that any imbalance in the loads would cause unequal phase shifts in each individual phase between the primary power source and the output load and this in turn would lead to an unbalanced three-phase output.
A three-phase uninterruptible power supply must be capable in general of supplying unbalanced loads. Therefore a proper control system of a three-phase uninterruptible power supply must be capable of maintaining balanced output voltage phase angles in the presence of unbalanced loads and in addition, must be capable of drawing power only from the primary power source during normal operation permitting the reserve power source to idle.